If you like Google Inbox and you're wondering why it doesn't replace Gmail, there's a setting that redirects Gmail to Inbox. Just go to Inbox, open the settings box, switch to the "Other" tab, enable "Redirect Gmail to inbox.google.com" and click "Done". The next time you type mail.google.com, gmail.com or click on a Gmail shortcut or bookmark, you'll be sent to inbox.google.com.
You can still open Gmail: just click the new "Gmail" menu item, which is placed below "Contacts" in Inbox's sidebar. The URL that temporarily disables redirects is http://ift.tt/1gwKrwv.
"If you often go to Gmail in your web browser when you really mean to go to Inbox, you can turn on a setting so that you'll automatically be taken to Inbox when you visit gmail.com or mail.google.com on a computer. You can always get back to Gmail using a simple link in Inbox," informs Google. The setting was added a few weeks ago.
